Whey is the complex waste of the dairy industry. Despite the fact, that it has numerous applications (like different form of food supplements), its major amount is still handled as waste. The carbohydrate, protein and lactic acid content, as well as the COD and BOD, are sufficiently high warranting disposal as waste resulting in high costs; however, their levels are insufficient for the cost-effective isolation and purification. Most of the numerous reports on whey utilisation focus on lactose utilization, while lactic acid removal is complex, but necessary, particularly in case of sour whey decontamination. According to our best knowledge among the microbial fermentation, the only lactic acid (as carbon source) utilization process is propionic acid fermentation. Propionic acid is an attractive product with a wide application range. In this study, two propionic acid producing microorganisms were investigated in terms of industrial applicability. The propionic acid producing bacteria are generally characterized by anaerobic metabolism (except the pathogenic P. acne); but, for application in a biorefinery, facultative anaerobe behavior is the most appropriate and cost-effective. In this study, the aero-tolerances of Propionibacterium freudenreichii subsp. shermanii and Propionibacterium acidipropionici were examined; their propionic acid-producing properties (yield, concentration, substrate preference, productivity) were compared.